The present invention relates to an improved spectrophotometer.
A conventional a spectrophotometer is adapted to split incident light into a plurality of light components of different wavelengths and measure the light intensity of each wavelength. These spectrophotometers calculate the sum of the products of the isochromatic function (x.lambda., y.lambda., z.lambda.) of CIE standard colorimetric system with the respective measured spectral values to convert the measured spectral values into trisimulus values.
However, in the case of measuring color density of a colored picture, for example, other spectral sensitivities than the isochromatic function are used. Moreover, there may be individuals who want to use their own spectral sensitivities in evaluating color. In such a case, it would be of great benefit if operators can set their own spectral sensitivities (referred to as user's spectral sensitivities) for color evaluation.
Conventionally, elements consisting of a band-pass interference filter array in combination with a photodiode array have been used as spectrodetectors for measuring the light intensity of wavelengths separated at equal intervals. The respective band-pass interference filters are subject to deviation of a few nm due to the manufacturing process used to make them. So, it is impossible to make the intervals of wavelengths exactly equal. Moreover, the values of wavelengths having peak spectral sensitivity often deviate in some degree due to errors caused by the positional relation between the filter array and the photodiode array.
As described above, wavelengths at which outputs from spectrodetectors peak do not have completely equal intervals to each other due to the errors caused during the manufacturing process. However, measured spectral values for wavelengths at equal intervals are often required. A small pitch of the order of 10 nm is often required as a pitch between each adjacent wavelength for the respective peak output. As a result, the output from such a conventional spectrodetector can not be used without regulation.
In depositing an optical film using a vacuum deposition device, the spectral reflectance of the optical film is measured in real time. In order to carry out the measurement, a film thickness monitoring device of the following type is proposed as shown in Japanese Patent Laid-open Publicatin No. 59-20804. In the monitoring device, light emitted from an illumination light source irradiates a test piece. Its reflected light is detected by a spectrodetector while the intensity of the emitted light of the light source is measured by one light receiving element. This prior art device has only one light receiving element for measuring light from the light source; and measures only luminosity of the light source. Variations in spectral energy distribution of the light source for illumination can not be measured by this device.
Use of a pulse xenon lamp for the light source may be preferable, because less electric power is required. The pulse xenon lamp, however, has a spectral energy distribution that is variable in each flash and therefore, the measured spectral value is subject to errors.
A spectrophotometer of the following type is known. Incident light is split into its components according to their wavelength by an interference filter, a spectral prism or a diffraction grating. A light receiving element is disposed a position corresponding to, each wavelength. The output from the light receiving elements provide the measurement result.
The conventional spectrophotometer, however, has disadvantages. The splitting means used, such as a spectral prism, diffraction grating or the like, do not always lead a light component to a light receiving element corresponding to the wavelength of the light component. Therefore, there is a possibility that a light receiving element will also receive light components of other wavelengths. For example, assume a light receiving element is disposed to measure a light component of 500 nm in wavelength. The splitting means conducts a light component of 500 nm, and may also leak light components of other wavelengths to the same light receiving element. The importance of this disadvantage increases or decreases with the degree of resolvability of the splitting means, possibility of the occurrence of stray light in the optical system, the half-width of the light receiving element, etc.
As can be understood from the above, it is impossible to obtain a correct spectral measurement with a conventional spectrophotometer because the output of each of the light receiving elements includes errors, so the output of each light receiving element is not identical with the measured light component of corresponding wavelength.